"The one thing that we're going to be doing is working closely with the Ministry of Labour, trying to determine exactly how that stage came down, and we're urging any witnesses that were in the area to come forward," Const. Tony Vella said at an early evening press conference on the grounds. "It's going to take us a large amount of time to determine exactly what occurred." He urged witnesses to contact Toronto police.

The Live Nation-promoted concert was scheduled to begin with openers Caribou at 7:30 p.m., followed by Radiohead at 8:30. No concert-goers had yet entered the grounds. The weather was sunny, clear and calm.

On Sunday morning, BBC News identified the deceased as 33-year-old British drum technician Scott Johnson. He sustained a "crushing injury" after being trapped under debris.

"The roof part of the stage collapsed," Toronto Fire Services spokesman Captain Mike Strapko told Reuters. "It's like an arch made out of round piping similar to what they use for scaffolding," he said, adding that the structure was rigged with lighting and other equipment. "So that's what came down and did crush the one individual."

"From our understanding, at this point in the investigation, there were a number of people on the stage preparing for a concert," Vella said. "They were setting up when the top portion of the stage collapsed on top of them. Unfortunately, four people were hurt [including the dead man]. The remainder of the people, when they heard the stage coming down, ran from the area."

Peter Rotolo, EMS Commander and onsite commander for this incident, said emergency personnel were already onsite for the concert when, at approximately 4 p.m., they responded immediately to the collapse, treating the injured staff.

"Unfortunately, one patient was pronounced [dead] at the scene by the paramedics, in conjunction with the coroner's office and Sunnybrook base hospital - and together with Toronto Fire Services, Toronto Police Services collectively we looked after the other patients that approached us, and others who came to us after the fact," Rotolo said. "One patient was transported to Sunnybrook with non-life-threatening injuries. Two were treated at the scene and were not transported to the hospital. We are remaining on the scene for a standby purpose for the investigation along with Toronto Fire services and Police Services and the Coroner's office."

Vella said questions relating to what kind of permits were required and obtained; the exact number of people on and around the stage; who build the stage; who owned it; whether charges were pending, and other details could not be answered at the time of the press conference. However, several sources told Billboard.biz that the stage was provided by Optex Staging and assembled by crew from event-staffing company Nasco.

(At press time, Live Nation, the venue and the band had not commented on the incident beyond issuing a statement that the concert had been cancelled and refunds and further information will be forthcoming.)

"The big question is how [the collapse occured]," Vella said. "That is something we will be working closely with the Ministry of Labour to determine exactly how this occurred to prevent any future cases.

"We're still at the early stages at this point, so we still need to conduct a full investigation before we can say anything else. Again, there are a number of officers on the scene; they're speaking with a number of people that were on the stage that managed to escape at the time, as well the forensics are on the scene taking photographs. So it's going to take some time. Once we have all the details, we will have a better understanding."